"can i just close the mbp to make it sleep and plug the external off without ejecting it first?"
No -- you always want to eject the external device/drive first.
If you don't use care in doing this, you _might_ end up with directory damage on the external drive, and sometimes they become un-recognizable to the Mac (won't mount up again).
Wake it up first, eject the drive, then put it back to sleep.....
BTW, I would also recommend that you do a "full restart" of the Mac at least once every couple of days. Nothing like "waking up fresh"!